Image sensors have become ubiquitous. They are widely used in digital still cameras, cellular phones, security cameras, medical, automobile, and other applications. The technology used to manufacture image sensors, and in particular CMOS image sensors, has continued to advance at great pace. For example, the demands of higher resolution and lower power consumption have encouraged the further miniaturization and integration of the image sensor.
Typically each pixel of an image sensor includes a photosensitive element, such as a photodiode, and one or more transistors for reading out the signal from the photosensitive element. With greater integration, the transistors are generally made smaller. For example, a transfer transistor is commonly used in a pixel using a four-transistor design. The transfer transistor has a transfer gate formed between the photosensitive element and a floating node. The transfer gate is an important element in the pixel and it is desirable to scale the transfer gate to have a shorter gate length for reasons of greater integration and enhanced pixel fill factor.
However, short gate lengths may result in leakage current from the photosensitive element to the floating node. One method of dealing with this leakage current is to increase the enhancement implant under the transfer gate. This increases the barrier/well potentials at the transfer gate and photodiode interface. The increase of the barrier/well potentials will also degrade image sensor performance by increasing image lag.